This invention relates to a receiving circuit. More particularly, it relates to a receiving circuit which compares an input digital signal with a reference voltage thereby to discriminate the pulse signal and to provide a pulse signal with its waveform regenerated.
A receiving circuit for digital signals typically includes means to regenerate the incoming pulse signal which has been distorted in the course of transmission, to its original signal waveform. In order to perform the regeneration operation precisely, it is a common practice to extract a timing signal from the received pulse signals and to regenerate the received signals while synchronizing them with the timing signal.
However, it is sometimes desirable to use a receiving circuit which is operable irrespective of the bit rate of the digital signals or to take steps in the design of the circuit arrangement or apparatus to make it simple and economical. In this regard, there has been known a receiving circuit which discriminates whether the level of the input pulse signal is greater or smaller than a reference value and which regenerates as the output pulse signal a signal having a pulse height that is not smaller than the reference value. In this case, when the reference voltage is fixed, the width of the regenerated pulse varies due to the distortion of the input pulse signal, especially the fluctuation of the level, and the variation of the pulse width forms a cause for the malfunction of a signal processing circuit disposed at a succeeding stage. Particularly, in the case of optical communication, a photodetector undergoes a temperature variation and the transmission is carried out at a high bit rate, so that the variation of the width of the regenerated pulse causes a bit error to drastically degrade the quality of the communication.
Heretofore, in order to solve such a problem, it has been proposed to set the reference voltage by detecting and holding the peak value of the input pulse and dividing the detected peak value into the reference voltage. This prior art method is effective, but inconveniences occur in the case where a preprocessing circuit, such as photodetector and preamplifier, is disposed in the front part of the receiving circuit. By way of example, in the case where one-half of the peak value of a high level corresponding to an input signal "1" is used as the reference voltage (threshold level), when the input potential of a signal "0" does not become zero or fluctuates due to the dark current of the photodetector, etc., the reference voltage might become lower than the input voltage corresponding to the signal "0". Moreover, the width of the pulse of the signal "1" which ought to become equal to or greater than the reference voltage might vary due to the fluctuation of the input voltage corresponding to the signal "0".